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Animal experiment. Researcher examining a white laboratory rat during research into its biological clock or circadian rhythm. During the experiment the rat is kept inside one of the light-proof chambers (at upper left). The levels of light in the chambers during the day can be controlled by the researcher, using the switches at lower left. A creature's biological clock controls the time at which it is active or asleep. It will respond to stimuli such as different light levels during the day. This research will help to find ways of ensuring the alertness of human night workers, and help find a cure for sleep disorders.